This invention relates to a visual output device which uses electro-optic cells such as liquid crystal cells. Such output devices are commonly used in calculators, watches, electronic thermometers, or the like.
With the advent of hand-held devices came the need for effective visual output means which could be powered using a low voltage. The liquid crystal cell has been widely accepted in the industry due to its ability to be constantly visible to the user, its low cost, its low voltage generation, and its relatively low power demand. An electro-optic cell may be characterized as a device which will selectively transmit light depending on the electrical energy applied. Commonly these cells use a liquid crystal material. Alternatives include, e.g.,electrophoretic cells.
There are many applications where it would be highly desirable to have a flat display with no gray scale. The gray scale available with a CRT is not necessary for the many applications where only character and graphic display is required. For example, most of the video output uses of a small computer could be accommodated by such a grahics display. However, for many of these applications a recently large display area (such as a thousand pixels are better) is desirable.
For information content higher than the 23 segments in a standard digital watch display, multiplexing the display is essential to avoid excessive chip/display interconnections. Multiplexing involves exciting one pixel and moving quickly to another operation. Given the X-Y matrix used in multiplexing, a given pixel receives excitation signals during the whole addressing cycle, not just at the time it is specifically addressed. In order for the drive technique to work, the pixel in question should not respond to the extra signals. This means that the electro-optic response (transmission function) of the device must have a threshold characteristic. The extent to which the display turns on in a multiplex drive scheme is related to the steepness or slope of the transmission vs. voltage curve above threshold. The sharper this transition, the more lines which can be addressed and consequently the more data that can be presented. For many nonemissive type displays, e.g., liquid crystals, the sharpness of this transfer function is poor, leading to a limited level of multiplexing and a limited information content. A general reference on matrix-addressed LCD arrays is Alt & Pleshko, "Scanning Limitations of Liquid-Crystal Displays," 21 IEEE Transactions on Electron Devices 146 (1974), which is hereby incorporated by reference.